1. Field of the Invention
The present invention relates generally to nuclear medical imaging, and in particular relates to scintillation detectors used in Positron Emission Tomography (PET).
2. Description of the Related Art
Silicon photomultipliers (SiPMs) can be used to improve the timing performance of positron emission tomography (PET) detectors over that achievable with conventional vacuum tube photomultipliers (PMTs). Some of the properties of these devices contributing to this are the high photon detection efficiency (PDE), a combination of high microcell fill factor and very high quantum efficiency (QE), high uniformity of microcell gain and good single photon time response. The small size of SiPMs allows finer sampling of pixelated or monolithic scintillator blocks, even down to one-to-one coupling of light sensor and scintillator pixel. This can reduce the light spread to the sensors required for event positioning which can result in significantly lower light path length and path length variance, and thus reduce timing jitter for low level leading edge discriminator timing methods. In addition, the SiPM shape (typically square or rectangular) and high active area percentage results in much higher geometric fill factor on a scintillator block than can be achieved with PMTs. The resulting increase in light collection efficiency improves initial photon collection and thus timing performance.
One possible method of exploiting these facts is to optically couple each scintillator pixel in a detector block to one SiPM. However, a number of practical and technical issues make this both difficult and sub-optimal for total PET system performance. A PET system using such 1:1 coupling would have to have tens of thousands of high timing performance, high power electronics channels. Scatter of 511 keV gammas between scintillator pixels is frequent, and although a system design might recover adjacent coincident events adding to the total energy, this would add to complexity and high electronic density and power, and some timing resolution loss for these events is inevitable. Cost effective and reliable monolithic arrays of SiPMs and arrays of discrete SiPMs in multi-device packaging have optical coupling between devices in the packaging optical window, which can degrade timing performance seen in one device/pixel channel. Optical reflector materials between every pixel result in light losses from the high aspect ratio of the pixel (due to the many reflections) which can lead to timing degradation, and lower packing fraction of scintillator, causing a loss of detective solid angle and thus system sensitivity reduction. These problems are not admitted to have been known in the prior art by inclusion in this section.
Information relevant to attempts to address these problems can be found in the following references, which are not admitted to be prior art with respect to the present invention by inclusion in this section:                (1) M. Conti et. al., “Estimating image quality for future generations of TOF PET scanners,” 2011 IEEE Nuclear Science Symposium Conference Record MIC9.S-25;        (2) C. Piemonte et. al, “Timing performance of large area SiPMs coupled to LYSO using dark noise compensation methods,” NSS/MIC IEEE 2011;        (3) A. Gola et. al, “Analog circuit for timing measurements with large area SiPMs coupled to LYSO crystals,” NSS/MIC IEEE 2011;        (4) G. F. Knoll, Radiation Detection and Measurement, 2nd Edition. John Wiley & Sons, Inc., 2010, pages 65-104;        (5) U.S. Pat. No. 7,019,297 B2 “Detector Array Using Internalized Light Sharing and Air Coupling”;        (6) U.S. Pat. No. 7,408,164 B2 “Detector Array Utilizing Air Gaps as a Reflector Between Array Elements”;        (7) US 2011/0210255 A1 “Multiplexing Readout Scheme for a Gamma Ray Detector”;        (8) US 2011/0017916 A1 “Reflector and Light Collimator Arrangement for Improved Light Collection in Scintillation Detectors”;        (9) US 2012/0061576 A1 “PET Detector System with Improved Capabilities for Quantification”; and        (10) US 2012/0199748 A1 “Radiation Conversion Elements with Reflectors for Radiological Imaging Apparatus.”        
A need exists to construct optically coupled arrays of scintillators on SiPM arrays such that the coincidence resolving time and total energy event capture sensitivity are optimized together—that is, to design small detector blocks or “miniblocks,” that are large enough to capture a high fraction of events without scatter losses outside them, while maintaining the best timing performance possible. The remaining smaller fraction of 511 keV events that still scatter between these miniblocks would then need to be recovered by a practical method of detecting full photopeak energy events while generating the best timing available from the individual miniblock signals.